116 ANIMAL PHYSIOLOGY. 
the vessel-walls, which, it will be remembered, are differentia- 
ted from the cells of the mesoblast, and, it may well be as- 
sumed, have not at this early stage entirely lost their amceboid 
character. The blood-vessels certainly have a respiratory func- 
tion, and suffice, till the more complicated villi are formed. 
The latter structures are in the main similar in build to the 
villi of the alimentary tract, and are adapted to being sur- 
rounded by similar structures of maternal origin. Both the 
maternal crypts and the foetal villi are, though complementary 
in shape, all but identical in minute structure in most in- 
stances. In each case the blood-vessels are covered superfi- 
cially by cells which we can not help thinking are essential in 
nutrition. The villi are both nutritive and respiratory. It is 
no more difficult to understand their function than that of the 
cells of the endoderm of a polyp, or the epithelial coverings of 
lungs or gills. ‘és 
Experiment proves that there is a respiratory interchange 
of gases between the maternal and foetal blood which nowhere 
mingle physically. The same law holds in the respiration of 
the foetus as in the mammals. Oxygen passes to the region 
where there is least of it, and likewise carbonic anhydride. If 
the mother be asphyxiated so is the fetus, and indeed more 
rapidly than if its own umbilical vessels be tied, for the mater- 
nal blood in the first instance abstracts the oxygen from that 
of the foetus when the tension of this gas becomes lower in the 
maternal than in the foetal blood; the usual course of affairs 
is reversed, and the mother satisfies the oxygen hunger of her 
own blood and tissues by withdrawing that which she recently 
supplied to the foetus. It will be seen, then, that the embryo is 
from the first a parasite. This explains that exhaustion which 
pregnancy, and especially a series of gestations, entails. True, 
nature usually for the time meets the demand by an excess of 
nutritive energy: hence many persons are never so vigorous in 
appearance as when in this condition; often, however, to be fol- 
lowed by corresponding emaciation and senescence. The full 
and frequent respirations, the bounding pulse, are succeeded by 
reverse conditions; action and reaction are alike present in the 
animate and inanimate worlds. Moreover, it falls to the parent 
to eliminate not only the waste of its own organism but that of 
the foetus ; and not infrequently in the human subject the over- 
wrought excretory organs, especially the kidneys, fail, entailing 
disastrous consequences. 
The digestive functions of the embryo are naturally inact- 
